Electronically scanned arrays (ESAs) with ultra-wideband (UWB) and wide-scan radiation performance are desirable for applications such as multi-functional systems, high-throughput or low-power communications, high-resolution and clutter resilient radar/sensing, and electromagnetic warfare systems. To this day, the most extensively utilized UWB-ESA element is the Vivaldi, or tapered-slot or flared-notch antenna, due to its excellent impedance performance. Vivaldi arrays are capable of achieving instantaneous bandwidths (defined as the ratio of the highest frequency to the lowest frequency) in excess of three octaves (>8:1). Several prominent embodiments of Vivaldi arrays have been realized over the past decade, including microstrip/stripline variants that using high-volume printed circuit board (PCB) manufacturing, and all-metal versions for high-power handling synthesized through electrical discharge machining (EDM) or additive manufacturing (3D printing) technologies.
Despite their excellent impedance performance at such wide bandwidths, all Vivaldi arrays are known to suffer from significant degradation of polarization isolation when scanning in the non-principal planes, especially at the diagonal planes. This is particularly problematic as the radiation energy instead of being carried in the intended radiation polarization (co-polarization) it is distributed in a polarization that is orthogonal to the intended one (cross-polarization) as the array scans away from the broadside and the principal radiation planes (E-/H-planes). This unintended polarization distortion causes polarization mismatch between the polarization vectors of the receiving antenna/array ({circumflex over (p)}a) and the transmitting antenna/array ({circumflex over (p)}tr) leading to loss of service or reduction of throughputs in communication scenarios because the polarization loss factor (PLF)PLF=|{circumflex over (p)}a·{circumflex over (p)}tr|2 in the Friis propagation equation approaches zero. Similarly for a radar scenario where the antenna/array is monostatic (transmitter/receiver are co-located), the polarization mismatch (or polarization isolation) of incident and scattered returns would succumb to high losses and may reduce the detection range. Similarly, in polarometric radar poor polarization isolation could reduce accuracy, target identification or clutter reduction capabilities. Therefore, in the absence of polarization correctional measures, significant losses incur as a consequence of high PLF that effectively inhibit operation when scanning off-axis in the diagonal planes. Polarization correctional procedures based on the re-adjustment of the array's excitation are known to achieve acceptable cross-polarization rejection in the diagonal planes, but they are only available to dual-polarized configurations could require additional feeding circuitry responsible for producing frequency-dependent amplitude/phase weights to each orthogonal feed. In addition to an added complexity and implementation cost, these look-up-table (LUT) based polarization corrections methods are scan angle and frequency-dependent and are inherently narrow beam and narrowband thus inhibiting the UWB instantaneous bandwidth potential of the Vivaldi array in the off-axis diagonal planes. As a result, Vivaldi antenna arrays have intrinsic restrictions when scanning in the diagonal planes that limit their performance. Another significant disadvantage of the LUT-based polarization correction approach is the unintended increase in polarization side-lobes.
It is believed that root cause of this off-axis diagonal plane scanning polarization purity degradation in Vivaldi array stems from the high profile of the array that is otherwise necessary for good impedance matching at the lower frequency band. An intrinsic bandwidth and polarization isolation design trade-off is thus engendered in scanned Vivaldi arrays, limiting effective scan volume or instantaneous bandwidth in Vivaldi. It is noted that this bandwidth and polarization isolation trade-off becomes more pronounced as the Vivaldi array design become more wideband, i.e. a Vivaldi array with 4:1 bandwidth has approximately 10 dB polarization isolation when scanned 45 degrees in the D-plane, but a 7:1 array has only 0 dB polarization isolation, respectively.
As a means to improve non-principal plane scanning polarization isolation of UWB-ESAs, low-profile vertically-integrated radiators such as the bunny ear antenna, bunny ear combline antenna (BECA), and balanced antipodal Vivaldi antenna (BAVA) have been proposed. The radiating conductors of each antenna incorporate flared dipole-like fins on the order of λhigh/2 that resemble miniaturized versions of a tapered slot from a Vivaldi antenna. These antennas are capable of achieving good polarization isolation but at the expense of bandwidth or/and matching level. The maximum documented instantaneous bandwidth achieved by these types of arrays is from a modified BAVA, termed U-channel BAVA array attaining a decade bandwidth (10:1), but at VSWR<3 for broadside with VSWR rising above 4 in H-plane 45 degree scanning. For well-matched bandwidths (broadside VSWR<2), comparable to those produced by Vivaldi arrays, typical values for said arrays range from 3:1 to 6:1, with some requiring external baluns that complicate high-volume fabrication.
Thus, a need remains for an antenna element that exhibits very large instantaneous bandwidths (>6:1) while maintaining excellent impedance matching (VSWR<2) and good polarization isolation in all non-principal scanning planes, including the diagonal one (better or equal than 15 dB at an elevation angle of 45 degrees).